Just got that answer right now:
If you add a new search engine, go to Settings > Basic > Search Engine > Manage search engines and click Add.
Next, you should: Enter the name of the search engine (Ecosia, for example).
Choose the keyword you would like to use as a shortcut in the search bar (e, for example).
Paste the URL with %s in place of the query,
which means that instead of https://www.ecosia.org/?c=en
you need to have http://ecosia.com/search?q=%s
For now, you can type the keyword shortcut in your search bar, followed by a space and your query, to search with your new search engine.
You can also select the search engine’s icon from the drop-down list on your search bar.We hope this helps
Opera Customer Support Team